Arc lamps have been used in many applications, including camera strobes, analytical instrumentation, surgical illumination, theatrical lighting, and laser and machine vision. In spite of the availability of other more convenient and low cost light sources such as LEDs (light emitting diodes), Arc lamps are still currently used in some niche areas because they have certain unique properties that other light sources cannot provide. These include high brightness, high power, high UV (ultra violet) light content, a wide continuous spectral distribution with excellent color balance and spectral flatness in the visible region, long life, and stable spectrum over life.
Arc lamps have two operation modes, namely DC and pulsed mode. The DC operation mode generally has a better arc stability and substantially longer life. However, this mode of operation is not ideal for photography, which only needs a short flash of illumination light while a photo is being taken. As for the pulsed mode of operation, the combination of wide spectrum and color balance with the ability to produce short pulses of high brightness light has made Arc lamps particularly suitable for biological photography, enabling excellent color projection and high-quality flesh tones. In this respect, short-arc flash lamps with an arc spacing of typically 1-3 mm are especially unique because they can provide pulses of high intensity and brightness light that other light sources cannot match. The high brightness and intensity is particularly desirable for superior camera performance. In addition, a short-arc flash lamp can also solve the problem of motion of a living biological sample such as a human eye and hence eliminate blurring of the obtained image. Furthermore, the wide spectral distribution of Arc flash lamps also makes them ideal for applications requiring light in specific spectral regions, such as red-free images and Fluorescein Angiography. The specific spectral region can be selected by placing different types of optical filter in the illumination and/or detection light path.
In spite of its wide use in biological imaging applications, the structure of an Arc lamp makes it difficult to efficiently collect the emitted light and deliver it to the object to be imaged with excellent uniformity. In general, the discharge formed between the two Arc lamp electrodes is a wandering but relatively linear light source. An ellipsoidal reflector can be used for relatively high efficiency light collection, and in such a case the lamp is usually aligned along the optical axis, with the arc at the first focus. A fiber optic bundle is usually placed with its input end at the second focus to collect the light. Although this arrangement focuses a relatively large portion of the available light from the arc, since the electrode will obstruct the light at angles of less than about 30 degrees from the optical axis, there is little light converging into the fiber bundle at angles close to the optical axis. This will cause problems in terms of light illumination uniformity for eye fundus imaging.
On the other side of the fiber optic bundle, the fiber bundle emitted light needs to be projected uniformly onto the object. Traditionally, this is done by some combination of conventional condensing lenses and/or spherical mirrors. For digital eye fundus imaging or photography, such a projection arrangement is not ideal. Commonly assigned U.S. Pat. No. 6,921,169 discloses an eye camera for imaging the retina. As pointed out in that patent, it is desirable to have a ring illumination beam to be co-axially projected on the eye lens so that illumination light is not reflected from the cornea, lens, and other parts of the eye to cause flair in the image. Further, the light beam must have a uniform as well as a wide enough angular distribution such that when the illumination beam is projected onto the retina of the eye, not only the central area of retina will be illuminated, but also the peripheral area of the retina can be substantially illuminated. However, existing ring beam projection schemes failed to achieve this in a cost effective way.
In a traditional ophthalmic instrument such as a photographic fundus camera, the illumination source is generally a combination light source module consisting of a continuous light source, typically a Halogen lamp, and a flash lamp, which is generally an Arc lamp. Either a removable beam splitter/mirror is used to combine the two light beams, or the filament of the continuous light source is imaged onto the gap between the two electrodes of the flash lamp to make them appear as one. The continuous light source provides aiming illumination for the photographer/imager, while the flash light source provides high energy pulse needed for high quality photography. The same type of configuration is widely used in both photographic fundus cameras for the posterior and slit lamp cameras for the anterior chamber of the eye. Such a module is complicated and expensive as it needs to combine two different light sources.